Currently, in the manufacturing process of metal oxide semiconductor (MOS), a lightly doped drain (LDD) is often used to prevent the short channel effect. Please refer to FIGS. 1a.about.1f which are schematic diagrams showing a conventional process for manufacturing an MOS transistor with a lightly doped drain. In FIG. 1a, a field oxide 103, a gate oxide 101, and a polysilicon layer 102 (serving as a gate) are formed on the silicon substrate 100 in sequence. Thereafter, a silicon dioxide 104 is formed over the silicon substrate 100 and the gate 102 by thermal oxidation as shown in FIG. 1b and then an ion implantation is performed to form a lightly doped drain 105 in the silicon substrate 100 by using the gate 102 as a mask, shown in FIG. 1c. In FIG. 1d, a silicon dioxide layer 106 is deposited on the surface of the wafer by plasma enhanced chemical vapor deposition (PECVD) and then etched by an anisotropic etch to form spacers 107 alongside the gate 102 (shown in FIG. 1e). The spacers can be used as a mask for the subsequent heavily doped implantation. Finally, after heavily doping the lightly doped drain 105, a lightly doped region 108 and a heavily doped region 109 are formed as shown in FIG. 1f and serve as a drain and a source.
However, during the etching process for forming the spacers, an over-etched situation may be happened, thereby resulting in a loss of the surfaces of the field oxide 103 and the lightly doped drain structure 1059 (designated by an imaginary line shown in FIG. 1e). For a small-size device, a shallow junction may prevent the short channel effect and punch through problem, but the above-described etching process for forming the spacers will seriously detract from the advantage of the shallow junction.
In addition, the size of the field oxide must be maintained to a certain degree in order to obtain a better isolation. However, this will cause that the device's size can not be effectively decreased. Therefore, it is tried by the Applicant to deal with the defects encountered by the prior art.